1. Field of the Invention
The present invention relates to melt processible fluoropolymer compositions in which inorganic fine particles are dispersed at the primary particle level. More specifically, the present invention relates to a melt processible fluoropolymer composition obtained by melt-mixing a melt processible fluoropolymer and aggregated inorganic fine particles formed by the cohesive force of the inorganic fine particles, wherein the inorganic fine particles are dispersed in the fluoropolymer at the primary particle level.
2. Description of Related Art
Melt processible fluoropolymers such as tetrafluoroethylene/perfluoro(alkyl vinyl ether) (PFA), tetrafluoroethylene/hexafluoropropylene (FEP), tetrafluoroethylene/ethylene (ETFE), have excellent heat resistance, chemical resistance, and low coefficients of friction. However, these fluoropolymers, and the perfluoropolymers in particular because of the weak mutual interaction between the molecules, can have deficiencies in dynamic physical properties (tensile strength, elongation) and dimensional stability under certain conditions; hence a fluoropolymer composition material with superior dynamic physical properties and dimensional stability is needed.
Conventionally, when fluoropolymer compositions with high performance are necessary, improvements in properties such as mechanical strength and dimensional stability are made by dispersing fillers in the polymer. In particular, the method of improving the mechanical characteristics by melt-mixing polymer with an organically treated clay, and then dispersing and breaking up the clay to the nano level, has been employed widely in recent years.
For example, in U.S. Pat. No. 5,962,553, nanocomposite comprising a organically-modified clay (organo-clay) and a fluoropolymer is disclosed. However, in this method there is a problem that a portion of the organic agent used to expand the interlayer space of the clay decomposes at the melt-mixing temperature of the melt processible fluoropolymer, which has high melting point, such as PFA and FEP. Also, in applications that require purity, such as semiconductor device manufacturing, the organic agent constitutes an impurity, and hence it poses problems.
The published Japanese Patent Application 2001-152030 describes a polymer composition, and the manufacture of the same, characterized by the fact that an additive selected from metals, metal salts, and inorganic compounds or a flame retardant is applied in advance to an inorganic porous body of average particle size 100 nm-1000 nm obtained by sintering an inorganic material such as porous glass or silicon dioxide (hereinafter this may be referred to as silica); this is mixed with a molten polymer so as to pulverize the inorganic porous body, and particles with the aforementioned additive or flame retardant of average particle size 10 nm-100 nm are dispersed in the polymer. However, the porous glass described in the gazette contains covalently bonded of silicon and oxygen; significant energy is necessary to pulverize and disperse the porous glass. Hence pulverizing and dispersing, porous glass mixing with molten polymer is very difficult. Also, in an inorganic porous body of average particle size 100 nm-1000 nm made by sintering aggregated inorganic fine particles comprising silica fine particles of average primary particle size 12 nm at 600° C.-700° C., only the surface layers fuse slightly and bond with each other due to surface fusion of silica particles (or aggregated silica particles) during sintering, and solidify into a skeleton with firm bonding (Resources and Material, vol 118, p. 202, 2002). Hence even if melted and mixed with a polymer in a melt-mixing device, the average particle size of the inorganic porous body after melt-mixing with polystyrene (PS) is 290 nm, the particle size distribution is broad at 40 nm-100,000 nm (100 μm), and pulverizing to the level of the original primary particle is not successful (Papers of the 13th Symposium of High Polymer Materials, p. 10, 2003). In particular, in such melt-mixing with polystyrene polymer there is a noticeable deterioration in the dynamic physical properties due to the presence of many sintered aggregates of incompletely pulverized or unpulverized inorganic fine particles of particle size 10 μm or greater.
Also, when melt-mixing inorganic fine particles or inorganic nano particles (fine particles of nanometer dimensions) in a polymer, agglomeration of the fine particles occurs due to the attractive forces of the fine particles because of their high surface to volume ratio. Therefore, it is very difficult to disperse the nanoparticles as is at the nanolevel even if nanoparticles are directly melt-mixed with a polymer.
Furthermore, in recent attempts to manufacture polymer nanocomposites incorporating nanofillers such as carbon nanotubes or carbon nanofibers in a polymer, and to disperse the nanofillers in polymer by melt-mixing, the dispersed state of the nanofillers varies according to the polarity (hydrophilicity being a measure of polarity: more polar polymer is more hydrophilic; as polymer polarity decreases, the polymer becomes more hydrophobic) of the polymer. Uniform dispersion of the nanofillers was possible to some extent in polar polymers such as nitrile rubber. However, it was difficult to evenly disperse the carbon nanotubes in a hydrophobic polymer such as ethylene propylene rubber (EPDM) (Polymer Preprints, Japan, vol 52, p. 1785, 2003). Therefore, it is very difficult to directly disperse carbon nanotubes or other nanofillers in melt processible fluoropolymers such as tetrafluoroethylene/perfluoro(alkyl vinyl ether) (PFA), tetrafluoroethylene/hexafluoropropylene (FEP), which are more hydrophobic than ethylene propylene rubber (EPDM), in the melt-mixing process.